The invention relates to a controllable semiconductor structure having improved switching properties.
The literature describes numerous component structures referred to as JFET or MESFET, in which the conduction properties are controlled by the voltage-dependent expansion of one or more space-charge zones (pn transition in JFET, Schottky transition in MESFET). The base structure was first proposed by W. Schockley: A Unipolar `Field-Effect` Transistor, in the Proceedings of the I.R.E., 1952. In standard-technology conversions, as are described in W. von Munch, Einfuhrung in die Halbleitertechnologie [Introduction to Semiconductor Technology], Teubner, 1993, large parasitic capacitances (especially input capacitance and reverse-transfer or Miller capacitance) occur, leading to low limit frequencies in amplifiers and causing long switching times, and therefore large switching losses, in switching applications. This is also the case for high-blocking JFETs that operate according to the RESURF principle, for example, as described in U.S. Pat. No. 4,422,089; in these JFETs, the field-intensity peaks at the component surface are reduced by a suitable selection of the doping and depth of the lateral drift zone.
It is known from textbooks, e.g., R. Paul: Elektronische Halbleiterbauelemente [Electronic Semiconductor Components] that JFETs and MESFETs are usually produced on, for example, insulating, semiinsulating or insulated substrates (e.g., the SOI technique or sapphire in silicon, highly-compensated material in gallium arsenide, etc.) to minimize the parasitic capacitances.
These techniques have the following disadvantages:
1) Because of the insulating, semi-insulating or insulated substrate, no current flow can occur in the vertical direction. Therefore, no vertical components can be produced with this method, which limits its use for power components. PA1 2) The production of wafers with an insulating or insulated substrate is complicated and expensive. In addition, problems due to, for example, temperature limitations can occur in the further processing. PA1 3) In semiconductors that cannot be rendered semi-insulating through compensation, a second material must be used as an insulator. This leads to, on the one hand, stress because of different thermal expansion coefficients and, on the other hand, more intense internal heating of the components because of the generally lower thermal conductivity of the insulator. Furthermore, the crystal quality of the active semiconductor layer is frequently worse in heteroepitaxial production on an insulator than in homoepitaxially-produced layers because of erroneous lattice adaptation. PA1 4) The insulation technique can only be combined with the RESURF technique in thin insulator layers, which in turn increases the parasitic capacitances.
It is therefore the object of the invention to use simple technological measures and few steps to create a semiconductor structure that has a good blocking effect, and permits higher limit frequencies and lower switching losses than conventional components.